Method of making reactive transformers



May 3, 1966 H. T. JONES METHOD OF MAKING REACTIVE TRANSFORMERS OriginalFiled Feb. 27, 1961 2 Sheets-Sheet 1 R. 05 mm w v m N. O M A m m H 0 6 42 H m "A 2 5 Purit- May 3, 1966 H. T. JONES METHOD OF MAKING REACTIVETRANSFORMERS 2 Sheets-Sheet 2 Original Filed Feb. 27, 1961 INVENTOR.Howard T. Jones His Attorneu United States Patent 3,248,781 METHOD OFMAKING REACTIVE TRANSFORMERS Howard T. Jones, Hendersonville, N.C.,assignor to General Electric Company, a corporation of New York Originalapplication Feb. 27, 19M, Ser. No. 91,730. Divided and this appiicationJan. 25, 1965, Ser. No.

2 Claims. c1. 29-15557 the lamp. In order to provide the currentlimiting action,

such transformers are conventionally of the high leakage reactance type,that is, they are provided with magnetic shunts providing a lowreluctance path for leakage reactance flux. The reactive transformerconstruction disclosed herein, although intended primarily forballasting mercury vapor discharge lamps and described principally fromthe standpoint of such utility, are also useful in related fields likeare welding and voltage regulation wherein it is desired to limit thecurrent through a load.

The transformer portion of a regulator ballast can be made in manyconceivable ways. The basic requirements are that independent primaryand secondary electrical circuits be mutually linked by a magneticcircuit and that another magnetic circuit be provided to link either theprimary or the secondary circuit without necessarily linking the other.In addition, the secondary electrical circuit should be linked by asaturable magnetic circuit wherein the vfiux linkages are not a linearfunction of the primary voltage.

In the past, reactive transformers have generally been provided with amagnetic core, formed of a stack of rel- .atively thin flat laminationsof magnetic material. The use of grain-oriented magnetic material ortransformer iron permits greater flux density with less core loss andlower exciting current. However when grain-oriented sheet magneticmaterial is used for trans-former core lamination, although the flux canbe parallel to the grain orientation in the principal direction of thecore, it is transverse to the orientation in the transversely arrangedportion of the core. Reactive transformers have also been made usingwound cores of grain-oriented magnetic material. The problem then arisesof making the maximum effective utilization of core winding techniquesto reduce the total amount of magnetic material required whilemaintaining superior electrical characteristics, and of providing inmost effective manner the shunts and other features necessary to achievethe desired electrical characteristics.

The object of the invention is to provide a reactive transformerconstruction and a method of making same wherein the most effectiveutilization is made of core and coils to reduce the total amount ofmaterial and labor or operations required while achieving superiorelectrical characteristics.

In a particular transformer construction and winding technique accordingto the present invention, the transformer core is formed by firstwinding a long strip of magnetic sheet material flatwise layer uponlayer to form 3,248,781 Patented May 3,1966

a laminated ring. The ring is then shaped by flattening to an elongatedstraight-sided loop, that is a loop forming an elongated generallyrectangular window. The core is then annealed for stress relief andgrain growth and may at this point be impregnated with a suitable resinor hinder whereby it will henceforth maintain its shape. The core isthen cut transversely by sawing or otherwise into substantially equalparts, each part having an elongated U-shape. Each part, which willhenceforth be referred to as a wound U-core, is now used as theprincipal portion of the core of a reactive transformer, so that fromthe origin-a1 wound core, two separate transformers will result.

To complete the reactive transformer, the U-core is provided with asaturable area of reduced cross section by sawing a slot partiallythrough the core normal to the edge of the lamination, preferably in'thebight or transverse portion of the U. Wound primary and secondaryelectrical windings are then assembled inductively on the U-core in sucha way that the secondary windings are at the closed end of the core loopwhere the restriction oc- An end member or yoke is placed across thelegs of the U to provide a closed magnetic circuit around a centralwindow in such way that the butt joints occur next to the primarywinding and are in the primary portion of the magnetic circuit.tReactive shunts are then introduced between the primary and secondaryportions of the core loop by placing a stack of laminations on one orboth sides of the core loop spanning the central opening so that thelaminations of the reactive shunts are edge to edge with those of theU-core. To prevent magnetic saturation of adjacent laminations, layersof paper or other composition may be inserted between the magneticshunts and the U-core. The yoke and the reactive shunts may be fastenedin place by a suitable strap.

A reactive transformer formed in the foregoing manner makes veryefficient use of magnetic material since all stock is initially in theform of strip, no irregular punchings productive of waste are used, andwinding and cutting operations are reduced to a minimum. The uniqueplacement of the component parts wherein the .air gap inevitablyintroduced by the yoke is located next to the primary windings andwherein the restriction is located next to the secondary windings,results in superior electrical characteristics under the limitationsunavoidably introduced by the winding technique. The invention thusprovides a comparatively inexpensive reactive transformer with nosacrifice in performance. In fact, the wound U- core reactivetransformers in accordance with the invention achieve a higherefficiency and show lower losses than conventional transformers using astack of punched l-aminations.

For further features and advantages andfor a better understanding of theinvention, attention is now directed to the following description ofpreferred embodiments and to the accompanying drawings. The features ofthe invention believed to be novel will be more particularly pointed outin the appended claims.

In the drawings:

FIGS. 1a and 11) illustrate, to a smaller scale than the other figuresof the drawings, the core of a transformer made according to theinvention at two different stages.

FIG. 2 is an isometric view of a loosely shunted transformer constructedin accordance with the invention.

FIG. 3 is a plan view, with the electrical windings sectioned, of thetransformer illustrated in FIG. 2.

FIG. 4 is an equivalent circuit for a regulator ballast for aredischarge lamps illustrating the electrical characteristics of areactive transformer according to the invention.

FIG. is a plan view similar to FIG. 3 but with the core structurepartially cut away and illustrating a variant of the present invention.

Referring to the drawings and more particularly to FIG. 1a, the corestructure of a reactive transformer according to the invention is madeby winding a long strip of grain-oriented magnetic sheet materialflatwise layer upon upon layer until a sufiicient cross-sectional areais built up and a laminated ring 1 is formed as shown. The laminatedring is then reformed, for instance by pressing in a suitable clampingjig or vise, to an elongated loop 2 having parallel sides defining arectangular window 3 as illustrated in FIG. 1b. Alternatively, the stripof magnetic sheet material may be wound directly on a suitable mandrelto a rectangular loop form. The laminations are next annealed by heattreatment to achieve stress relief and grain growth, the stress reliefpermitting the shape to be maintained; additionally, to preventsubsequent separation of the laminations, the loop may be impregnated atleast in part with a suitable resin or binder which is allowed to set orpolymerize. The rectangular loops is then out along a single plane 4, 4transverse to the long sides, into two equal U-shaped portions 2a, 2b.The loop may be severed by any suitable means such as a metal saw but,in order to prevent short circuits between laminations, it is preferredto use a milling cutter or a thin abrasive wheel.

In accordance with the invention, where a core type reactive transformeris to be made, one of the U-shaped parts 211, or 2b illustrated in FIG.1b is used as the main portion of the core. A transformer so constructedis illustrated in FIGS. 2 and 3. Before assembling the windings on theU-core, a restriction in the cross-sectional area of the core is createdin order to provide a saturable area in that portion of the core loopwhich is to carry the secondary winding. This is most conveniently donein the bight or transverse portion of the U by cutting a narrow slot 5across one edge of the lamination to a depth of approximately /3 thewidth of the lamination. Next prewound and preformed electrical windingsare slipped into place over the legs of the U-core. A pair of secondarywindings 6, 7 is slipped on first so as to be at the closed end of theU-core next to the slot 5. Then the primary windings 8, 9 are positionednear the open end of the U- core legs. The windings are preferablyinsulated from the core by a layer of insulating material such as thepaper insulators 10 shown.

The main magnetic circuit linking the primary and secondary winding iscompleted by means of a yoke 11 which is butted up against the ends ofthe legs of the U-core so as to bridge the gap. The laminations of theyoke are arranged as illustrated to meet edge to edge with those of theU-core but with the plane of the laminations in the yoke transverse tothat of the laminations in the U-core. An insulating layer 12a, suitablya thin sheet of paper, or a resin coating, or the oxide on thelaminations is interposed between the core and yoke at the butt joints12 to prevent a continuous electrical circuit around the core. Anadvantageous feature in accordance with the invenvention resides in thatthe butt joints 12 formed by juxtaposing yoke 11 to the ends of theU-core 2a occur in that part of the main magnetic circuit bearing theprimary windings. But joints are unavoidably present in the wound coreconstruction adopted and, by placing them in the primary portion of themain magnetic circuit, the losses which they introduce are kept to aminimum. The effective air gap at the butt joints affects the primarymagnetizing reactance and the gap may be adjusted, for instance byinserting additional paper separators, to control the input powerfactor, The laminations of the yoke are held together by means of frameplate 13 and the yoke is fastened to the U-core 2a by means of a steelstrap 14 whose ends are clamped together in conventional fashion by acrimping band 15.

In order to provide a magnetic circuit linking either the primary or thesecondary windings without necessarily linking them both together, amagnetic shunt is added to the basic core loop between primary andsecondary windings. As illustrated, the reactive shunt consists of apair of stacks of laminations 16, 17 which are spaced from thelaminations of the core by thin insulating spacers 18, 19. Thelaminations of the reactive shunt are placed against the side of themain magnetic lop in edge to edge relationship with the laminations ofthe main loop. The reactive shunts span the central opening through thecore, thus providing a leakage path for the primary and secondaryportions of the core loop. The entire reactive transformer assembly isordinarily mounted in a suitable container or can and impregnated with apotting compound; prior to such potting, the shunts may be held in placeon the core loop by tape (not shown) wound for example about the centeror ends of the stacks of laminations 16, 17. By placing the reactiveshunts consisting of stacks of laminations beside the wound core loopinstead of in the same plane, and by orienting its laminations edge toedge with those of the main core loop, an effective low loss shunt isobtained. The losses with this arrangement are substantially lower thanthey would be if the shunt were placed in the plane of the main coreloop which would entail that most of the shunted flux would necessarilyhave to pass through several adjacent layers of the laminations in themain core loop in a direction normal to the laminations.

A reactive transformer serving as a regulator ballast for a gaseousdischarge device such as a mercury vapor lamp or a fluorescent lamp, isintended to maintain a nearly constant current into the lamp load withvariations of supply potential. When the lamp is operated about at itsrated power, variations in lamp current are reflected in the energyoutput of the lamp even though the potential across it remains nearlyconstant. The regulator ballast performs its function by supplying asubstantially constant voltage to the lamp and by limiting the amount ofcurrent which the lamp is permitted to draw. The present reactivetransformer is intended to provide a non-linear transformer couplingbetween a source of power and a load circuit including a dischargedevice such as an arc lamp, and a series capacitor. The seriescapacitor, in addition to its current limiting function, provides thenecessary leading current through the non-linear transformer coupling sothat a constant output voltage may be obtained. The factors invloved maybe explained by reference to the schematic diagram of FIG. 4 showing theequivalent circuit of a reactive transformer according to the inventionconnected as a regulator ballast for an arc lamp 20. The elementsenclosed within the dashline rectangle represent the effective impedanceelements resulting from the transformer construction which has beendescribed. These elements include an ideal transformer 21 having a turnsratio of preselected value. Reactance 22 represents the primarymagnetizing reactance referred to the secondary circuit. Reactances 23and 24 represents, respectively, the secondary magnetizing reactance andthe total leakage reactance. The output circuit is completed by theconnection of a shunt capacitor 25 across the output leads and a seriescapacitor 26 in series with the load consisting of arc lamp 20.

With the transformer construction which has been described, it ispossible to vary each of the effective reactances to obtain optimumcharacteristics for a given installation. The reactance shunts 16, 17placed between the primary and secondary core provide the effectiveleakage reactance 24 which must be proportioned to the series capacitorreactance 26. The value of this leakage reactance may be adjustedwithout altering the other effective reactances, by varying the crosssectional area of the laminations in the reactive shunts and the widthof the air gap between the edges of the reactive shunts and the edges ofthe core loop. The effective secondary magnetizing reactance 23 providesa non-linear saturating effect that affects both the regulation of loadcurrent with variations in input potential and the crest factor of theload current. This reactance is controlled principally by the corerestriction placed on the secondary side of the main core loop. Theshape of the restriction, whether hole or slot, and also its dimensionscontrol the actual characteristics of the secondary magnetizingreactance. Normal variations in the primary supply voltage effect onlythe degree of saturation in the secondary magnetic circuit so that thesecondary magnetizing reactance remains substantially constant and thusthe voltage output of the ballast likewise remains nearly constant.

By placing the butt joints 12 in the primary portion of the core loop,they affect only the primary magnetizing reactantce 22, and theeffective gap may be adjusted to control the input power factor. If thebutt joints were located in the secondary portion of the core loop, theywould affect also the secondary magnetizing reactance, thereby adverselyaffecting regulation and increasing transformer losses. Shunt capacitor25, which may also be termed a peaking capacitor, is used to increasethe peak open circuit voltage above the turns ratio voltage and shouldpreferably be of a value to resonate with the leakage reactance 24 atslightly above the third harmonic frequency of the supply voltage.Although shown as connected across the entire output of the ballasttransformer, the peaking capacitor may also be connected across only apart of the secondary turns to reduce to some extent the open circuitvoltage supplied by the ballast.

A variant of the invention is shown in FIG. 5 illustrating a shell typereactive transformer. In this construction, a pair of U-cores 2a, 2b arepositioned side by side and the common or center leg of the resultingshell-type core is constituted by two juxtaposed legs, one from eachU-core. The single secondary winding 31 is slipped over the center legso as to be located next the closed or inner end of the U-cores. Thecore restriction, in this instance is provided by holes 32, 33 drilledthrough the transverse portions or bights of the U-cores, being therebylocated in the secondary portion of the main core loop. A single primarywinding 34 is slipped over the center leg so as to be located next tothe butt joints 35 where the open ends of the U-cores are bridged by ayoke 36 consisting of a stack of laminations. The plane of thelaminations in the yoke 36 is transverse to that of the lamiations inthe U-cores, in similar fashion to the embodiment of FIGS. 2 and 3, andthe laminations are likewise arranged for edge to edge contact. Thereactive shunt is provided by two stacks of lamiantions, only one ofwhich, 37, is shown in the figure, spanning the windows of both coreloops and located between the primary and secondary windings. Thereactive shunts are arranged in similar fashion to those in FIGS. 2 and3, being placed edge to edge on the side of the main core loop.

The shell type reactive transformer construction of FIG. 5 makes use oftwo core loops but only one set of primary and secondary windings.Evidently, for a given transformer rating, the cross sectional areas ofthe two U-cores which make up the complete core will be equal to halfthat of the core of the transformer in FIGS. 2 and 3. Thus in thisconstruction, both parts 2a, 2b of an elongated rectangular core loop asillustrated in FIG. 1b are used in a single reactive transformer buteach core part is of half the cross sectional area. The shell type coreconstruction offers most of the features of the transformer designillustrated in FIGS. 2 and 3 along with the added advantage of animproved outward heat transfer from the core because of a greaterexposed core area.

Although certain preferred embodiments have been illustrated anddescribed herein, it is to be understood that these are intended asexemplary and not limitative of the invention. Modifications inconstruction and method will readily occur within the scope of thepresent teachings, to competent designers. The appended claims areintended to cover any such modifications falling within the true spiritand scope of the inventions. A

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. The method of making two reactive transformers comprising winding aband of magnetic material flatwise layer upon layer to form an elongatedstraight-sided loop, cutting said loop in two midway through theelongated sides to form a pair of substantially equal sized U-shapedcore portions having parallel straight legs, and proceeding thereafterwith both said U-shaped core portions to produce two transformers byplacing on the legs of each said U-shape core portion primary andsecondary electrical windings so that the core legs project beyond saidwindings at the open core end, closing the open end of each saidU-shaped portion by a yoke formed of an individual flat stack oflaminations placed with their edges abutting the end edges of those ofthe U-shaped portion and with their planes at right angles thereto, andplacing on each said U-shaped portion an individual reactive shuntformed of at least one fiat stack of laminations of magnetic materialagainst the side of each leg of the U- shaped portion between saidprimary and secondary electrical windings, the laminations of each legof the U- shaped portion and of said individual reactive shunt beingplaced in edge-to-edge relationship and with their planes at rightangles to each other.

2. The method of making two reactive transformers comprising winding aband of magnetic material fiatwise layers upon layer to form anelongated straight-sided loop, cutting said loop in two midway throughthe elongated sides to form a pair of substantially equal-sized U-shapedcore portions having parallel straight legs, and proceeding thereafterwith both said U-shaped core portions to produce two transformers byplacing on the legs of each said U-shaped core portion secondarywindings next to the closed end and primary windings next to the openend so that the core legs project beyond said windings at the open end,closing the open end of each said U-shaped core portion by an individualyoke to provide a closed magnetic loop around a window, said individualyoke being formed of a flat stack of laminations placed with their edgesabutting the end edges of those of the U-shaped portion and with theirplanes at right angles therto whereby the junctures of the laminationsof the U-shaped portion with those of the yoke form butt joints next tosaid primary windings, and placing on each said U-shaped portion areactive shunt formed of at least one flat stack of laminations ofmagnetic material against the side of the U-shaped portion so as to spansaid window, the laminations of the U-shaped portion and of saidindividual reactive shunt being placed in edge-to-edge relationship andwith their planes at right angles to each other.

References Cited by the Examiner UNITED STATES PATENTS 1,992,822 2/1935Granfield 336213 2,324,634 7/1943 McCreary 336- 3,128,443 4/1964 Hermanet a1. 336l60 FOREIGN PATENTS 1,198,453 12/ 1959 France.

WHITMORE A. WILTZ, Primary Examiner.

R. W. CHURCH, Assistant Examiner.

1. THE METHOD OF MAKING TWO REACTIVE TRANSFORMERS COMPRISING WINDING ABAND OF MAGNETIC MATERIAL FLATWISE LAYER UPON LAYER TO FORM AN ELONGATEDSTRAIGHT-SIDED LOOP, CUTTING SAID LOOP IN TWO MIDWAY THROUGH THEELONGATED SIDES TO FORM A PAIR OF SUBSTANTIALLY EQUAL SIZED U-SHAPEDCORE PORTIONS HAVING PARALLEL STRAIGHT LEGS, AND PROCEEDING THEREAFTERWITH BOTH SAID U-SHAPED CORE PORTIONS TO PRODUCE TWO TRANSFORMERS BYPLACING ON THE LEGS OF EACH AND U-SHAPE CORE PORTION PRIMARY ANDSECONDARY ELECTRICAL WINDINGS SO THAT THE CORE LEGS PROJECT BEYOND SAIDWINDINGS AT THE OPEN CORE END, CLOSING THE OPEN END OF EACH SAIDU-SHAPED PORTION BY A YOKE FORMED OF AN INDIVIDUAL FLAT STACK OFLAMINATIONS PLACED WITH THEIR EDGES ABUTTING THE END EDGES OF THOSE OFTHE U-SHAPED PORTION AND WITH THEIR PLANES AT RIGHT ANGLES THERETO, ANDPLACING ON EACH SAID U-SHAPED PORTION AN INDIVIDUAL REACTIVE SHUNTFORMED OF AT LEAST ONE FLAT STACK OF LAMINATIONS OF MAGNETIC MATERIALAGAINST THE SIDE OF EACH LEG OF THE USHAPED PORTION BETWEEN SAID PRIMARYAND SECONDARY ELECTRICAL WINDINGS, THE LAMINATIONS OF EACH LEG OF THEUSHAPED PORTION AND OF SAID INDIVIDUAL REACTIVE SHUNT BEING PLACED INEDGE-TO-EDGE RELATIONSHIP AND WITH THEIR PLANES AT RIGHT ANGLES TO EACHOTHER.